Colloquium, IIA, Bangalore, May 23, 2006

R. T. GangadharaIndian Institute of Astrophysics

Bangalore

PLAN• DISCOVERY OF PULSARS• PULSAR RADIO PROFILES• EMISSION MECHANISM

-Curvature Radiation• EMISSION HEIGHT:

-Aberration-Retardation-Polar cap current

DISCOVERY OF PULSARSThe first Pulsating Source of Radio (PSR) was discovered in August 1967 by Jocelyn Bell and Tony Hewish in Cambridge, England.

-Project set out to investigate interplanetary scintillation.

4Extremely regular pulses:Clock.

4Variable intensity:Interference or Scintillation.

4Appeared 4 minutes earliereach day:Therefore, celestial.

4Sinusoidal change of pulseperiod:Doppler shift due to EarthSun motion.

4Pulse duration ~ 0.02 sec:Source no larger than Earth.

PSR B1919+21Freq. = 81.5 MHzPeriod = 1.337 sec

PROPERTIES:-

NATURE OF PULSARSWhat are the options?

A pulsating compact object ? White Dwarf or Neutron Star.- resonant periodicity > 1 sec for white dwarfs. - resonant periodicity > 1 ms for neutron stars.- doubtful to account for observed pulse stability but plausible.

Orbiting object ? Binary Stars or Small Satellites.

- gravitational radiation losses would decay the orbit on time scale ~ 2 days !- tidal disruption of satellite severe.

Intelligent extra-terrestrial sources ? Little Green Men !- transmitter power at stellar distances.- more than one source found !!

~ 1024 W

Rotating compact object ? Neutron Star.- simple analysis gives Pmin» 3p Gr 1�2, similar numbers to oscillation.

So, White Dwarfs and Neutron Stars are the prime candidates …Notable discoveries were the Crab (33 ms) and Vela (89 ms) pulsars.White Dwarf models were quickly ruled out on break-up grounds.

Neutron star phenomenon looks to be the clear winner !Hewish et al. 1968, Nature, 217, 709

PULSAR SPIN PERIOD

Conservation of angular momentum: Ms Rs2 Ws = M R2 WEx: Sun

Period ~ 27 days, Angular velocity Ws ~ 2.7x10- 6radsec- 1

If R = 10 km, then W= Ws Rs R 2~ 104 radsec- 1

Periodofrotation P = 2 p W~ 0.6millisec

Observed range of Pulsar periods: 1.57 millisec to 5.1 sec

Radius Rs = 6.96x108 m

Animation of Crab pulsar spinning and emitting two beams of radiation

P

Period:P= 2 p�W

LIGHT HOUSE MODEL OF PULSAR

SOUNDS OF RADIO PULSARS

Light Curve

Inte

nsit

y

Single and Average pulses of PSR B0329+54Spin period = 0.71 sec

PULSE PHASE (bins]

Radio Freq. = 606 MHz

Pulse intensity fluctuations

PSR B0950+08Pulse-to-pulse fluctuations

Hankins & Cordes (1981)

Rankin (1983)PULSAR EMISSION BEAM IS CONAL

PULSAR EMISSION BEAM IS PATCHY Lyne & Manchester (1988)

Pulse profiles from the radio to gamma--rays

NUV profile is similar to optical (Kern et al. 2003).

-May be same emission mechanism.

Soft x-ray:-Shallow pulsations with a sine-like light curve.-Single broad maximum per pulse.

Thermal emission from the surface of NSWith non-uniform temp. distr.

Optical and NUV pulse str. Indicates non-thermalemission.

Supernova Explosion of a massive star leads to the formation of neutron star.

The Crab Nebula Pulsar•only 1000 years old!

•still inside its supernova remnant

•emits a pulsar wind and jets

•produces visible pulses –only young pulsars have enough energy to do this

•20-km in diameter and is spinning at 30 rps

Magnetic dipole radiation of rapidly rotating neutron star

Crab nebula is powered by magnetic dipole radiation emitted by a rapidly rotating and highly magnetized neutron star (Pacini 1967).

Rate of rotational energy loss : E = -d

dt 1

2 I W2 = - I WW

Power radiated by magnetic dipole : W =2

3 M¦2 W4

c3

Rate of change of angular velocity: W = - ik2 M2 sin2 a3 c3 I

y{ W3Braking index n is defined by W= k Wn

Pulsar age :

Pulsarmagneticfield: Bs = 3.2 ´ 1019 P P×

G

t = 1n - 1

PP

= P

2 P , for n = 3.

Pulsar Population

DNS binaries live here

Internal structure of Neutron star

Gangadhara, R. T. & Gupta, Y., 2001, ApJ, 555, 31

P = 0.71 s

How do they work?

(Sturrock 1971; Ruderman & Sutherland 1975)

V / c

Equation for a field line in the Cartesian coordinate system (x,y,z):

Equation for field line : r = re sin2 q

Magnetic field line

Field line constant : re

r® = x, y, z

= re sin3 q cosf , sin3 q sinf , sin2 q cosqc

(Gangadhara, R. T., 2005, ApJ, 628, 923)

g = 340, 390

PSR B0450-18

n = 318 MHz , p = 0.5489 s , a = 24° , b = 4°_____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)_____________________________________________________________

1 7.78±0.07 1.01±0.07 3.50±0.01 -43.98±0.30 236±17 0.20±0.00 284±0.37 1.00±0.01 _______________________________________________________________________________________

P1 = 1.13E-16 erg/s

rem r�rLC

Gupta, Y., & Gangadhara, R. T. 2003, ApJ, 584, 418

P1 = 1.13x10- 16erg�s

PSR B1237+25

n = 318 MHz , p = 1.3824 s , a = 53° , b = 0°____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)____________________________________________________________

1 1.84±0.07 0.28±0.07 0.98±0.05 89.45±0.02 161±40 0.19±0.01 382±5 1.00±0.092 3.70±0.05 0.72±0.05 1.97±0.03 88.89±0.02 414±29 0.24±0.00 415±2 0.85±0.033 6.04±0.04 0.93±0.04 3.21±0.02 88.18±0.01 535±23 0.19±0.00 384±1 0.99±0.02____________________________________________________________

P1 = 6.23 x10- 17erg�s

PSR B1237+25

n = 318 MHz , p = 1.3824 s , a = 53° , b = 0°____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)____________________________________________________________

1 1.84±0.07 0.28±0.07 0.98±0.05 89.45±0.02 161±40 0.19±0.01 382±5 1.00±0.092 3.70±0.05 0.72±0.05 1.97±0.03 88.89±0.02 414±29 0.24±0.00 415±2 0.85±0.033 6.04±0.04 0.93±0.04 3.21±0.02 88.18±0.01 535±23 0.19±0.00 384±1 0.99±0.02____________________________________________________________

P1 = 6.23 x10- 17erg�s

MILLISECOND PULSAR:PSR J0437-4715

Discovered in Parks by Johnston et al. (1993).

- Nearest and bright millisecond pulsar (~180 pc)

- Spin period 5.75 ms- Has a low mass white dwarf in binary with

orbital period 5.74 days

Data by Johnston and Manchester (2005) from Parks.

(Gil & Krawczyk 1997)

Phase

Sing

le p

ulse

s

CONCLUSION

® Duetogeometricrestrictions, observer receivesthecore inner emissionfrom lowerheightswhile theconalemissionfrom higherheights.

® Pulsar profiles become asymmetricdue toaberration,retardationandpolarcapcurrents.

® Radius - to - frequency mapping RFMcan be explained by considering curvatureradiation.

CONCLUSION

• Developed a method for estimating the absolute emission heights of core as well as conal components.

In millisecond pulsar: J0437-4715 (i) A/R phase shift is quite high (~20 deg).(ii) Core component is emitted from an altitude close

to NS surface (~20 Km).(iii) Emission altitude of cones increases from core

to outer most cone (~90 Km).

(Gil & Krawczyk 1997)PSR J0437-47151440 MHz

Km 48 r4

20

≈−=

=

β

α

lc tcoc

LCr/r2−≈′φδ

LCr/r2≈′φδ

GMRT RESULTS

• Millisecond Pulsar PSR J0514-4002 in globular cluster NGC 1851 - Rotational period of 4.99 ms- Orbital period of 18.8 days

• PSR J1833-1034 in supernova remnant G21.5-0.9 - Rotational period of 61.86 ms

Paulo et al., 2004, ApJ, 606L, 53

Gupta et al., 2005, Current Science, Submitted

Neutron Binary Stars

– Intense X-rays from neutron stars in binary systems There are several types of X-ray binaries

• X-ray bursters from gas falling on the neutron star

• X-ray pulsars from hot-spots on the neutron star• infalling gas can “spin up” an old neutron star

BINARY PULSARS

A Double Pulsar Binary System

Jodrell Bank Observatory, UKMultibeam detector Parks